Congestion-aware Packet Repetitions for IEEE 802.11bd-based Safety-critical V2V Communications

Reliable V2V communications is key to enable energy-efficient and accident-free road mobility. Packet repetitions as proposed for IEEE 802.11bd allow to increase the transmission reliability by combining redundant messages at the cost of increased channel load, leading to packet loss and rising access delays. As so far no investigations on this reliability trade-off are available, we present the first comprehensive analysis on repetitions for the new technology. Therefore, we quantify the combining gain in link-level simulations, which show a benefit of frequency diversity. Next, we model the repetition reliability trade-off in dependency of the vehicle density on system-level. The results show a severe performance degradation with increasing density caused by the added transmissions. Based on the results, we elaborate on how to design an optimal distributed repetition algorithm. It shows that defining static channel load thresholds is not sufficient to maximize the transmission reliability, as it does not correctly considers the impact of the allocation. Finally, we propose to employ multi-channel repetitions to mitigate potential channel congestion, meanwhile maximizing the combining gain.